Among the processes for treatment of waste water utilizing microbes are the process of suspended microbes as heretofore represented by activated sludge process, and the process of fixing the microbes by attaching them to stones, plates, plastics, etc. as encountered in the trickling filter, rotating disc filter, etc., and it is well known that all these processes have been in wide use. Further, in recent years there has been investigated a process utilizing fine particle carrier to which microbes have been attached. The most characteristic features in the application of fine particle carrier to the biological treatment are (1) that the concentration of the microbes may be taken to be extremely high, (2) that the specific surface area of the carrier becomes large, (3) that as the result the rate of reaction per unit volume of reactor can be increased so high that the viological reactor can be designed compact, (4) that especially when the microbial concentration has been taken to be high, it is very easy to separate the microbes from the liquid, although such a separation has heretofore been considerably difficult, and so on.